Question: In history class, the girl to boy ratio is $4$ to $6$. If there are a total of $40$ students, how many girls are there?
A ratio of $4$ girls to $6$ boys means that a set of $10$ students will have $4$ girls and $6$ boys. A class of $40$ students has $4$ sets of $10$ students. Because we know that there are $4$ girls in each set of $10$ students, the class must have $4$ groups of $4$ girls each. There is a total of $16$ girls in history class.